Interpretation of coronal off-limb spectral line width measurements

Abstract

Spectral line widths measured above the solar limb have been used to infer wave velocity amplitude height dependencies, supporting Alfven wave coronal heating and wave driven solar wind models. Comparison of the inferred amplitude/density dependence with a constant magnetic field strength, undamped wave propagation model have shown good agreement, providing the first strong support of Alfven wave heating models. We derive the density dependence for wave propagation in an expanding flux tube with decreasing field strength, and show that the resulting density dependence is identical to that of the constant field strength case. We use the expanding flux tube propagation model to infer the wave amplitude in the low photosphere, which is a factor of 9 lower than granulation velocities.